overhead sign support structures: meeting aashto 2001 john w. van de lindt cdot staff bridge...

Overhead Sign Support Overhead Sign Support Structures: Meeting AASHTO Structures: Meeting AASHTO 20012001

John W. van de LindtJohn W. van de Lindt

CDOT Staff Bridge Communication Day – September 27, 2004

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MotivationMotivation


AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001

Fatigue problems

Where to begin ?

Project ObjectivesProject Objectives

Develop a method to analyze and evaluate overhead sign support structures in order to determine a metric accounting for both cost and performance.

Apply the method to overhead sign support structure designs currently in use in the U.S.

Check the identified structures for compliance with AASHTO 2001


Nationwide SurveyNationwide Survey

State of the Art / State of the Practice Survey

Brief and Simple (9 Questions / 15 min.)

What types of sign structures are being used?

Are there any that are having problems?

Is the AASHTO 2001 Sign Specification being used?


Survey ResponseSurvey Response

38 Responses = 76 % CDOT Staff Bridge Communication Day – September 27, 2004

Survey HighlightsSurvey Highlights

Great variety in different types of sign structures

20 states claim to be using AASHTO 2001 already

Steel is the dominant material used


Cost AnalysisCost AnalysisCost data is not easily available and highly variable

Steel weight is controlling factor in production cost

Variability in construction / fabrication methods

• Constructability factors Tubular = 1.00 Monotube = 1.15 Truss = 1.20

Cost Parameter = Steel Weight

Constructability Factor


Performance AnalysisPerformance AnalysisEstimated fatigue life of structural connections subject to natural wind gust loading

Fatigue design was the focus of AASHTO Sign Specification update

Existing work identified structural connections as susceptible to fatigue problems

All types of structures are susceptible to natural wind gust loading

Random vibration approach – Crandall and Mark (1961)


Structure Modeling - FEAStructure Modeling - FEA Simplified FEA model to determine dynamic properties

MDOT Cantilever Simplified Model

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Modeling -ConnectionsModeling -Connections

Connections modeled according to cross-sectional properties

MDOT Cantilever Base MDOT Cantilever Arm - PoleCDOT Staff Bridge Communication Day – September 27, 2004

Wind Loading StatisticsWind Loading StatisticsWind Velocity Histogram

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Wind speed distribution over the contiguous USA in the 1980’s (NOAA)CDOT Staff Bridge Communication Day – September 27, 2004

Wind Loading Statistics – Wind Loading Statistics – Data FittingData Fitting

Comparison of NOAA to lognormal PDF

Lognormal

NOAA


Wind Loading Statistics Wind Loading Statistics - Probabilities- Probabilities

• 25 bins for 25 wind velocities

• Area of BIN = Probability of Occurrence (Poi)

Dynamic Analysis - Dynamic Analysis - LoadingLoading

• Convert wind velocity to force (AASHTO 2001/ASCE7)

20.00256 z w r d signF K GV I C A• Initial conditions for free vibration

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Initial Position

Initial Velocity

Initial Acceleration

• Stiffness and Damping Matrices

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Rayleigh DampingCDOT Staff Bridge Communication Day – September 27, 2004

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Stress – Time HistoryStress – Time History

Standard deviation of combined stress = i

Fatigue Constants – Fatigue Constants – S-NS-N CurvesCurves• Stress category from AASHTO 2001 Sign Specification

• S-N curve from AASHTO 1994 LRFD Bridge Specification

bNS c

# of Cycles to Failure

Magnitude of Stress

Fatigue Constants


Fatigue LifeFatigue Life

• Damage from each stress time history (25)

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• Fatigue life from all damages

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Performance Parameter = Estimated Fatigue Life


Cost Utility FunctionsCost Utility Functions• Convert cost and performance into common units

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Cost Utility


Performance Utility Performance Utility FunctionsFunctions

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Performance Utility


Combining UtilitiesCombining Utilities

( , ) 1.0c pR c p a U c a U p

• Weighting factors

Adjust emphasis of cost (ac) and performance (ap)

1c pa a


Ranking Results – Excluding Ranking Results – Excluding CostCost

Index Ranking Index Ranking Number State Description Parameter Rank Number State Description Parameter Rank

5 IN 1-Arm 0.999 1 3 AK Monotube 0.999 17 MI 2-Arm C 0.999 2 4 CO Monotube 0.999 14 CO Monotube 0.998 3 5 IN 1-Arm 0.999 19 MI 2-Arm E 0.980 4 11 MO 2-Arm Truss 0.999 18 MI 2-Arm D 0.931 5 13 CA 4-Chord Truss 0.999 16 IN 2-Arm 0.854 6 15 MN 4-Chord Truss 0.999 115 MN 4-Chord Truss 0.831 7 16 WI 4-Chord Truss 0.999 116 WI 4-Chord Truss 0.528 8 8 MI 2-Arm D 0.999 211 MO 2-Arm Truss 0.423 9 6 IN 2-Arm 0.999 32 AK 2-Arm 0.346 10 9 MI 2-Arm E 0.997 41 AK 1-Arm 0.295 11 1 AK 1-Arm 0.996 514 FL 3-Chord Truss 0.277 12 2 AK 2-Arm 0.978 613 CA 4-Chord Truss 0.201 13 7 MI 2-Arm C 0.922 73 AK Monotube 0.181 14 10 MO 1-Arm 0.788 810 MO 1-Arm 0.161 15 12 WV 2-Arm Truss 0.551 912 WV 2-Arm Truss 0.118 16 14 FL 3-Chord Truss 0.095 10

17 CO Monotube 0.727 1 22 MO 2-Arm Truss 0.997 124 SD 2-Arm Truss 0.408 2 20 MI 4-Chord Truss 0.989 219 FL 3-Chord Truss 0.330 3 23 OR 4-Chord Truss 0.201 321 MN 4-Chord Truss 0.240 4 19 FL 3-Chord Truss 0.095 422 MO 2-Arm Truss 0.139 5 24 SD 2-Arm Truss 0.095 420 MI 4-Chord Truss 0.095 6 17 CO Monotube 0.072 518 CA Monotube 0.072 7 18 CA Monotube 0.072 523 OR 4-Chord Truss 0.072 7 21 MN 4-Chord Truss 0.072 5

Bridge/Span Sign Structures Bridge/Span Sign Structures

POLE-BASE ARM-POLE

Cantilever Sign Structures Cantilever Sign Structures


Ranking Results – 25% CostRanking Results – 25% Cost

Index Ranking Index Ranking Number State Description Parameter Rank Number State Description Parameter Rank

7 MI 2-Arm C 0.970 1 1 AK 1-Arm 0.977 15 IN 1-Arm 0.938 2 11 MO 2-Arm Truss 0.973 29 MI 2-Arm E 0.927 3 8 MI 2-Arm D 0.959 38 MI 2-Arm D 0.907 4 9 MI 2-Arm E 0.939 44 CO Monotube 0.902 5 5 IN 1-Arm 0.938 56 IN 2-Arm 0.752 6 2 AK 2-Arm 0.928 615 MN 4-Chord Truss 0.643 7 16 WI 4-Chord Truss 0.917 716 WI 4-Chord Truss 0.563 8 7 MI 2-Arm C 0.911 811 MO 2-Arm Truss 0.540 9 4 CO Monotube 0.903 92 AK 2-Arm 0.455 10 3 AK Monotube 0.876 101 AK 1-Arm 0.451 11 6 IN 2-Arm 0.861 1110 MO 1-Arm 0.370 12 10 MO 1-Arm 0.841 1214 FL 3-Chord Truss 0.352 13 15 MN 4-Chord Truss 0.770 1312 WV 2-Arm Truss 0.306 14 13 CA 4-Chord Truss 0.750 143 AK Monotube 0.262 15 12 WV 2-Arm Truss 0.631 1513 CA 4-Chord Truss 0.151 16 14 FL 3-Chord Truss 0.215 16

17 CO Monotube 0.699 1 22 MO 2-Arm Truss 0.998 124 SD 2-Arm Truss 0.508 2 20 MI 4-Chord Truss 0.853 222 MO 2-Arm Truss 0.354 3 24 SD 2-Arm Truss 0.273 321 MN 4-Chord Truss 0.256 4 17 CO Monotube 0.208 419 FL 3-Chord Truss 0.247 5 18 CA Monotube 0.206 518 CA Monotube 0.206 6 23 OR 4-Chord Truss 0.153 620 MI 4-Chord Truss 0.182 7 21 MN 4-Chord Truss 0.129 723 OR 4-Chord Truss 0.056 8 19 FL 3-Chord Truss 0.071 8

Cantilever Sign Structures Cantilever Sign Structures

Bridge/Span Sign Structures Bridge/Span Sign Structures

POLE-BASE ARM-POLE


OH Signs SelectedOH Signs Selected

MDOTMDOT– Optimal OH sign support Optimal OH sign support

structurestructure Bridge Type

Michigan Type C-70ftMichigan Type C-100ft

Michigan Type DFlorida 3-Chord Truss

Minnesota 4-Chord TrussMissouri 2-Arm

Optimization of Cost and Performance of Overhead Sign Support Structures (Ahlborn et al, 2003)

Cantilevered

Michigan Type CMichigan Type DMichigan Type E

Indiana 2-ArmColorado Monotube

Wisconsin 4-Chord Truss


GENERAL OVERVIEWGENERAL OVERVIEW

Design Check ProcedureDesign Check Procedure

– Structural analysis methodStructural analysis method Elastic methodElastic method Allowable stress design (ASD)Allowable stress design (ASD)

– Structural propertiesStructural properties


GENERAL OVERVIEW GENERAL OVERVIEW (CONT.)(CONT.)

Design Check ProcedureDesign Check Procedure

– Serviceability requirementsServiceability requirements Not consideredNot considered

– Wind and ice loadsWind and ice loads Michigan’s locationMichigan’s location

– Steel and Fatigue design checksSteel and Fatigue design checks Fatigue not considered for bridge typesFatigue not considered for bridge types


LOADSLOADS

Excerpted from AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001.

D + Ice

tc(Wha, Wp, Wvc)

tc(Wha, Wp, Wvc)

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Side view Normalview

Top view

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LOADS (CONT.)LOADS (CONT.)

Michigan Type C Michigan Type C CantileveredCantilevered– Group II-Case 2– Visual Analysis

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LOAD COMBINATIONSLOAD COMBINATIONSAbbreviation Name Load Combination

G1 Group I DL

G2C1 Group II, Case 1 DL + W

G2C2 Group II, Case 2 DL + W

G3C1 Group III, Case 1 DL + Ice + 1/2 W

G3C2 Group III, Case 2 DL + Ice + 1/2 W

G4G* Group IV, Galloping Fatigue

G4N* Group IV, Natural Wind Gust Fatigue

G4T* Group IV, Truck Induced Wind Gust Fatigue

*This load case only applied to cantilevers.

Note : Fatigue load combinations can be applied to both cantilevered and

bridge type OH sign support structures. Refer to NCHRP Report

494 for more information on fatigue on bridge types.

LOAD CASE Normal Component Transverse Component

1 1.0 (W ) 0.2 (W )2 0.6 (W ) 0.3 (W )

Excerpted from AASHTO Standard Specifications for Highway Signs, Luminaires, and Traffic Signals, 2001.


CALCULATED CALCULATED STRESSESSTRESSES Stress ResultantsStress Resultants

– PPxx, P, Pyy, P, Pzz, M, Mxx, M, Myy, M, Mzz

Connection Connection PropertiesProperties– A = area of patternA = area of pattern– c = distance from c = distance from

centroid to point*centroid to point*– I = moment of I = moment of

inertia of patterninertia of pattern– J = polar moment J = polar moment

of inertia of patternof inertia of pattern

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Michigan Type C Michigan Type C CantileveredCantilevered– Base-to-Column Bolts


ALLOWABLE ALLOWABLE STRESSESSTRESSES

Calculated according to 2001 Calculated according to 2001 AASHTO design codeAASHTO design code– Anchor boltsAnchor bolts

– Other bolts and all weldsOther bolts and all welds ReferencesReferences

– AASHTO AASHTO Standard Specifications for Standard Specifications for Highway BridgesHighway Bridges

– AWS AWS Structural Welding Code D1.1-Structural Welding Code D1.1-SteelSteel


RESULTSRESULTSStructure Type of OH Did the structure Description of Design

Description Sign Support meet AASHTO 2001? Shortcoming

Michigan Type C Cantilevered YES NAMichigan Type D Cantilevered YES NAMichigan Type E Cantilevered YES NAColorado Monotube Cantilevered YES NAIndiana 2-arm Cantilevered NO Base welds in fatigue.Wisconsin 4-chord truss Cantilevered NO Base bolts, base welds,

arm-to-column bolts, andarm-to-column welds allin fatigue.

Michigan Type C-70ft Bridge YES NAMichigan Type C-100ft Bridge YES NAMichigan Type D Bridge YES NAMinnesota 4-chord truss Bridge NO Chord splice welds under

load case G2C1.Florida 3-chord truss Bridge YES NAMissouri 2-arm truss Bridge YES NA


CONCLUSIONCONCLUSIONSS

RecommendationsRecommendations– Inelastic method vs. elastic methodInelastic method vs. elastic method– More accurate and detailed FEM’sMore accurate and detailed FEM’s

Future WorkFuture Work– DOT’sDOT’s

Adopt typical plans Adopt typical plans Adopt 2001 AASHTO design codeAdopt 2001 AASHTO design code

– Monitor OH sign support structuresMonitor OH sign support structures

– Design and researchDesign and research Serviceability requirementsServiceability requirements Include fatigue in design checks for bridge typesInclude fatigue in design checks for bridge types


Thank You!Thank You!

My Contact Information:

John W. van de LindtAssociate ProfessorColorado State UniversityDepartment of Civil EngineeringFort Collins, CO 80523-1372Ph: 970-491-6605 or 970-491-8691Fax: 970-491-7727E-mail: [email protected] Both reports are available in PDF at:http://www.engr.colostate.edu/~jwv/reports.htm


overhead sign support structures: meeting aashto 2001 john w. van de lindt cdot staff bridge...

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